Path-integrated Measurements Research Articles

Origin and Acceleration of the Slow Solar Wind

This Letter uses Doppler dimming measurements by SOHO UVCS to elucidate the origin and acceleration of the slow solar wind. By investigating plasma flow in the corona over an active region during 2000 May 14-16, we confirm what has been suggested by the presence of the imprint of active regions in the solar wind near Earth orbit, that active regions are a source of slow wind. The observed active region does not have an associated streamer in the outer corona. We explain how this implies that any related heliospheric current sheet must be transverse to the line of sight. It is this favorable geometry of a transverse heliospheric current sheet that allows the plasma flow over the active region to be isolated in path-integrated Doppler dimming measurements. The results also show that acceleration of the slow wind associated with active regions toward its terminal speed is faster than that along the heliospheric current sheet. These differences in acceleration explain why the signatures of the heliospheric current sheet are dissimilar in velocity, but not in density, between the corona and solar wind measured near Earth orbit.

Novel Algorithm for Tomographic Reconstruction of Atmospheric Chemicals with Sparse Sampling

Numerical studies were performed to evaluate a new air monitoring method for reconstructing chemical exposures and source emissions, based upon optical remote sensing (ORS) and computed tomography (CT). With an ORS-CT system, two-dimensional maps of chemical concentrations can be created that have good spatial and temporal resolution. The mathematical algorithm used to compute the distribution is critical for accurate and useable reconstructions of the concentrations. In this research, a novel reconstruction method was tested that uses maximum likelihood expectation maximization (MLEM) combined with two techniques called grid-translation and multi-grid (GT-MG). To evaluate this method, computer simulations were performed using 120 test maps of varying complexity and a simulated ORS system with four instruments and a total of 40 path-integrated measurements. The results were quantitatively compared with two previously used reconstruction methods (single-grid and grid-translation). Results using the GT-MG method were dramatically improved over previously used methods. Quantitatively, peak exposure errors were reduced by up to 85% and artifacts were dramatically minimized.

Path concentration profile reconstruction of optical remote sensing measurements using polynomial curve fitting procedures

Open path Fourier transform infrared (OP-FTIR) spectroscopy is one of several optical remote sensing (ORS) techniques that can identify and quantify many air pollutants. These instruments provide path-integrated measurements along the scanning beam path. There is a growing interest in gaining spatial resolution from ORS through innovative multiple-beam configurations and mathematical algorithms. In this study, we explored the use of a polynomial curve directly fitted to the path-integrated concentration (PIC) data for this application. The derivative of the fitted curve is the reconstructed density function of the underlying line concentration profile. We first validated this approach with a series of simulated Gaussian plumes. The PIC data were simulated by integrating the Gaussian functions over each beam path. We found that the piece-wise polynomial (cubic spline) algorithm performs better than the global polynomial algorithm. This approach was also applied to data collected previously in a ventilation chamber. Nitrous oxide was released from a point source as a tracer gas. A unistatic OP-FTIR collected the PIC data with 4 retroreflectors located along a one-dimensional domain for 15 min. For a wide plume, the concentration profile reconstructed from the OP-FTIR measurements fits well to the plume measured from 14 point samples collected simultaneously during the experiments ( R 2=0.83). For a narrow plume, the R 2 value between the point samples and the reconstructed plume is 0.40, similar to the one calculated from applying 4 ray-traced PIC. From the ray-traced synthetic data, it was also found that an additional PIC could improve the fitted R 2 value significantly to above 0.7.

OPTICAL PATTERNATION OF A WATER SPRAY USING STATISTICAL EXTINCTION TOMOGRAPHY

Tomographic reconstruction of local liquid surface areas per unit volume in a turbulent spray using path-integrated laser transmittance measurements is reported. Path-integrated transmittances from a turbulent spray were obtained using a newly developed six-axes optical patternator. A multiaxis maximum-likelihood estimation algorithm was developed to deconvolute for the local extinction coefficients from the path-integrated transmittances. The local extinction coefficients are identical to the local liquid surface area per un it volume within the spray. The algorithm was first evaluated using synthetic path-integrated data. The algorithm successfully recovered the local extinction coefficients from the synthetic path-integrated transmittances. The algorithm was then used to obtain local liquid surface areas per unit volume from the path-integrated transmittance measurements in the spray. The path-integrated extinction measurements were also compared with mass flux data obtained using a mechanical pattemator. The results suggest that the local velocity and size distribution of the spray field is highly correlated with the local surface area density in sprays.

Line profile reconstruction: validation and comparison of reconstruction methods

Currently, open path Fourier transform infrared (OP-FTIR) spectrometers have been applied in some fenceline monitoring, but their use has been limited because path-integrated concentration measurements typically only provide an estimate of the average concentration. We present a series of experiments that further explore the use of path-integrated measurements to reconstruct various pollutant distributions along a linear path. Our experiments were conducted in a ventilation chamber using an OP-FTIR instrument to monitor a tracer-gas release over a fenceline configuration. These experiments validate a line profile method (1-D reconstruction). Additionally, we expand current reconstruction techniques by applying the Bootstrap to our measurements. We compared our reconstruction results to our point samplers using the concordance correlation factor (CCF). Of the four different release types, three were successfully reconstructed with CCFs greater than 0.9. The difficult reconstruction involved a narrow release where the pollutant was limited to one segment of the segmented beampath. In general, of the three reconstruction methods employed, the average of the bootstrapped reconstructions was found to have the highest CCFs when compared to the point samplers. Furthermore, the bootstrap method was the most flexible and allowed a determination of the uncertainty surrounding our reconstructions.

Solar Wind Consequences of a Coronal Hole Density Profile: Spartan 201-03 Coronagraph and [ITAL]Ulysses[/ITAL] Observations from 1.15 [FORMULA][F]R[INF]⊙[/INF][/F][/FORMULA] to 4 AU

Spartan 201 is a small shuttle-launched and -retrieved satellite, whose mission is to study the origins of the solar wind. It carries on board two instruments, the Ultraviolet Coronal Spectrometer and the White-Light Coronagraph. The third mission of the Spartan 201 (1995 September 7-10) spacecraft was to provide a solar context for the in situ particles and fields measurements during the north polar passage of the Ulysses spacecraft. In this Letter, we characterize the physical conditions of the north polar coronal hole as derived from white-light coronal observations by the Spartan 201-03 White-Light Coronagraph, the ground-based K coronameter in Mauna Loa, Hawaii, and Ulysses observations of in situ particles and their velocity. For the first time, we are able to combine in situ and path-integrated measurements in a coronal hole, to yield a consistent electron density (N) profile from the Sun to the Earth and the outer heliosphere. By using the value of N measured by Ulysses (1.8-4 AU), we are able to determine the actual value of N and not just an upper limit in the polar coronal hole, near the Sun. The current N profile suggests that the acceleration of the fast solar wind in a coronal hole is complete by 10-15 R, much closer to the Sun than had been previously expected.

A new microwave tomography approach for rainfall monitoring over limited areas

Exploiting the relationship between microwave attenuation and rainfall intensity in a microwave tomography approach was demonstrated in the past to be a valid alternative to weather radar measurements for the reconstruction of rainfall fields in limited areas. This is useful in areas (e.g. valleys or small basins), where radar data are not available due to beam blocking or, in general, for real time monitoring of rainfall events occurring over zones characterized by a high hydrological risk. At each time step, path-integrated attenuation measurements are provided by a set of microwave transmitter-receiver pairs, and two-dimensional Gaussian basis functions are utilized to reconstruct the space-time distribution of the rainfall intensity field. The inversion problem to be solved is highly ill-conditioned, due to the practical (and economical) impossibility to set up an adequate network for performing classical tomography. Therefore a Global Optimization Stochastic Technique has been developed to obtain valid results in quasi real time. Some search parameters of the algorithm must be chosen depending on the network characteristics: this allows a kind of calibration depending on the adopted quality (complexity) of the network. Some investigations have been performed both on simulated rain fields and on radar monitored rainfall events. Some significant examples are presented, showing computational speed and flexibility of the technique, and that a network based on 10 stations can provide a estimate of the mean rainfall intensity over a basin of nearly 400 Km 2 with an error less than 10% even with a monitoring time of 30 seconds. Under the same conditions, it is possible to guess the rainfall distribution pattern. We show that, utilizing a longer observation time, more reliable estimates are obtained.

Open-Ocean Modeling as an Inverse Problem: The Primitive Equations

The ill-posedness of regional primitive equation models is examined, using a regional shallow-water model. The ill-posedness is resolved by reformulation as a least-squares inverse problem, in the sense that the Euler-Lagrange or variational boundary conditions ensure unique solutions for the linearized problem. The inverses for nonlinear problems are calculated using variants of simulated annealing and massively parallel computing. Simple experiments compare the relative merits of pointwise measurements and path-integrated measurements in compensating for bad boundary data. Error statistics are calculated, despite the large dimension of the state space.

Tomographic Reconstruction of Rainfall Fields through Microwave Attenuation Measurements

Abstract A new method is presented for estimating the space-time rainfall intensity distribution at ground level over a limited area. This is based on a tomographic technique that exploits the relationship between microwave attenuation and rainfall intensity. At each time step, the spatial distribution of rainfall intensity is calculated from a set of path-integrated microwave attenuation measurements, performed over the monitored area. A set of bilinear basis functions is used to reconstruct the rainfall space distribution from discrete samples. A recursive optimization technique is applied to solve the ill-conditioned system of linear algebraic equations providing the searched solution at each time stop, thus, obtaining an estimate of space-time distribution of rainfall intensity. This method is analyzed using computer simulated data, and various results are shown that validate the proposed tomographic technique. In particular a rainfall event is simulated over a 20 × 20 km2 plane area, assuming that pr...

Measurements on a shock wave generated by a solar flare

It is shown that path-integrated radio-scattering measurements of the solar wind velocity yield higher estimates than those of point measurements. The post-shock speed is determined from radio scattering measurements by selection of the maximum of the radio-scattering deduced from the velocity-time curve, whereas velocity points on the rising edge of the shock underestimate the post-shock gas velocity, and therefore the shock speed. However, it is suggested that large uncertainties reside in using radio techniques for measurements of parameters of the shock wave, and the uncertainties may be large enough to encompass the lower velocity values calculated by a point method of measurement.